1. Field of the Invention
The present invention relates to an apparatus and method for testing the performance of a sensing system including a rotatable wheel supporting an array of ferromagnetic teeth, and a variable reluctance detector positioned such that rotation of the wheel causes each of the teeth to pass the detector in turn.
2. Description of Related Arts
Anti-lock braking systems (ABS) are well known for preventing vehicle wheels locking during braking. ABS systems require an apparatus for measuring the speed of rotation of the braked wheel. and wheel speed signals are generally produced by a variable reluctance detector placed adjacent a series of ferromagnetic teeth supported by the rotating wheel. As each tooth passes the detector, an output is generated the amplitude of which is a function of the distance of the tooth from the detector and the speed of the tooth relative to the detector. The output is, of course, in phase with the movement of the tooth which causes it to be generated.
It is desirable to be able to test the signal generators incorporated in ABS systems. Tests must be designed which make it possible to detect a variety of faults, in particular an off-center wheel, an insufficient output, a damaged, dirty or missing tooth, or an incorrect number of teeth.
If the teeth are arranged in a circle which is not centered on the axis of rotation of the wheel, the distance between the teeth and the detector will vary with the angular position of the wheel. The amplitude of the detector output will vary with the variation in the distance between the teeth and the detector.
If the detector is not correctly positioned such that the minimum space between it and the teeth is too large, the detector output will be reduced in amplitude and as a result the system may become ineffective.
If a tooth is missing, the detector will produce an output which is of greater amplitude than would otherwise be the case but that output will have a period twice that which would normally be expected. A similar result would occur if the gap between two adjacent teeth was filled with dirt containing ferrous particles. A tooth which has been partly broken off will of course simply generate a smaller than expected output.
ABS systems are designed to operate with a predetermined number of teeth on the rotating wheel. It is clearly important to be able to check that a wheel being tested has the appropriate number of teeth.
The conventional approach to testing ABS systems has been to drive the wheel at a constant speed and monitor the detector output. An off-center wheel can then be detected by monitoring the amplitude of the detector output which will vary in a cyclical fashion over the course of one revolution of the wheel. Missing or damaged teeth can be detected by reference to the period of the detector output, a missing tooth being indicated by the detector output cycle having twice its normal period when the position of the missing tooth corresponds to the position of the detector. Other possible problems with the system can equally be tested providing appropriate care is taken in monitoring the detector output. Any anomalies measured can be attributed to the condition of the teeth and nothing else. Unfortunately, this approach to testing such systems is based on the assumption that the speed of rotation of the wheel will be constant. Any fluctuation in the wheel speed will result in fluctuations in the detector output. For example, varying speed will vary both the output period and the output amplitude. As a result, speed fluctuations can produce outputs from the detector which are similar to the outputs that would be expected in the case of faults that it is wished to detect, for example off-center wheels or missing teeth. For this reason it has not been thought possible to test such systems except when the wheels are turned by a constant speed rolling road. Service organizations not equipped with rolling roads have, however, tried to test ABS systems simply by manually rotating the wheel to be tested, but the results have not proved satisfactory because of the difficulty of achieving a steady wheel speed.